High selectivity polyimide membrane for natural gas upgrading and hydrogen purification

ABSTRACT

A polyimide polymer having hydroxyl and acetoxy function groups is provided together with a membrane made from the polymer. Also provided is a process for separating at least one gas from a mixture of gases using a polyimide membrane comprising a polyimide polymer with hydroxyl and acetoxy functional the process comprising: (a) providing the polyimide membrane comprising the polyimide polymer with hydroxyl and acetoxy functional groups which is permeable to the at least one gas; (b) contacting the mixture on one side of the polyimide membrane to cause said at least one gas to permeate the membrane; and (c) removing from the opposite side of the membrane a permeate gas composition comprising a portion of said at least one gas which permeated the polyimide membrane.

BACKGROUND OF THE INVENTION

This invention relates to a high selectivity polyimide membranecomprising a polyimide polymer with hydroxyl and acetoxy functionalgroups and methods for making and using the membrane.

In the past 30-35 years, the state of the art of polymer membrane-basedgas separation processes has evolved rapidly. Membrane-basedtechnologies have advantages of both low capital cost and high-energyefficiency compared to conventional separation methods. Membrane gasseparation is of special interest to petroleum producers and refiners,chemical companies, and industrial gas suppliers. Several applicationsof membrane gas separation have achieved commercial success, includingN₂ enrichment from air, carbon dioxide removal from natural gas and fromenhanced oil recovery, and also in hydrogen removal from nitrogen,methane, and argon in ammonia purge gas streams. For example, UOP'sSeparex™ cellulose acetate spiral wound polymeric membrane is currentlyan international market leader for carbon dioxide removal from naturalgas.

Polymers provide a range of properties including low cost, permeability,mechanical stability, and ease of processability that are important forgas separation. Glassy polymers (i.e., polymers at temperatures belowtheir T_(g)) have stiffer polymer backbones and therefore let smallermolecules such as hydrogen and helium pass through more quickly, whilelarger molecules such as hydrocarbons pass through more slowly ascompared to polymers with less stiff backbones. Cellulose acetate (CA)glassy polymer membranes are used extensively in gas separation.Currently, such CA membranes are used for natural gas upgrading,including the removal of carbon dioxide. Although CA membranes have manyadvantages, they are limited in a number of properties includingselectivity, permeability, and in chemical, thermal, and mechanicalstability.

The membranes most commonly used in commercial gas and liquid separationapplications are asymmetric polymeric membranes and have a thinnonporous selective skin layer that performs the separation. Separationis based on a solution-diffusion mechanism. This mechanism involvesmolecular-scale interactions of the permeating gas with the membranepolymer. The mechanism assumes that in a membrane having two opposingsurfaces, each component is sorbed by the membrane at one surface,transported by a gas concentration gradient, and desorbed at theopposing surface. According to this solution-diffusion model, themembrane performance in separating a given pair of gases (e.g., CO₂/CH₄,O₂/N₂, H₂/CH₄) is determined by two parameters: the permeabilitycoefficient (abbreviated hereinafter as permeability or P_(A)) and theselectivity (α_(A/B)). The P_(A) is the product of the gas flux and theselective skin layer thickness of the membrane, divided by the pressuredifference across the membrane. The α_(A/B) is the ratio of thepermeability coefficients of the two gases (α_(A/B)=P_(A)/P_(B)) whereP_(A) is the permeability of the more permeable gas and P_(B) is thepermeability of the less permeable gas. Gases can have high permeabilitycoefficients because of a high solubility coefficient, a high diffusioncoefficient, or because both coefficients are high. In general, thediffusion coefficient decreases while the solubility coefficientincreases with an increase in the molecular size of the gas. In highperformance polymer membranes, both high permeability and selectivityare desirable because higher permeability decreases the size of themembrane area required to treat a given volume of gas, therebydecreasing capital cost of membrane units, and because higherselectivity results in a higher purity product gas.

One of the components to be separated by a membrane must have asufficiently high permeance at the preferred conditions or anextraordinarily large membrane surface area is required to allowseparation of large amounts of gases or liquids. Permeance, measured inGas Permeation Units (GPU, 1 GPU=10⁻⁶ cm³ (STP)/cm² s (cm Hg)), is thepressure normalized flux and is equal to permeability divided by theskin layer thickness of the membrane. Commercially available gasseparation polymer membranes, such as CA, polyimide, and polysulfonemembranes formed by phase inversion and solvent exchange methods have anasymmetric integrally skinned membrane structure. Such membranes arecharacterized by a thin, dense, selectively semipermeable surface “skin”and a less dense void-containing (or porous), non-selective supportregion, with pore sizes ranging from large in the support region to verysmall proximate to the “skin”. However, fabrication of defect-free highselectivity asymmetric integrally skinned polyimide membranes isdifficult. The presence of nanopores or defects in the skin layerreduces the membrane selectivity. The high shrinkage of the polyimidemembrane on cloth substrate during membrane casting and drying processresults in unsuccessful fabrication of asymmetric integrally skinnedpolyimide flat sheet membranes using phase inversion technique.

US 2005/0268783 A1 disclosed chemically cross-linked polyimide hollowfiber membranes prepared from a monoesterified polymer followed by finalcross-linking after hollow fiber formation.

U.S. Pat. No. 7,485,173 disclosed UV cross-linked mixed matrix membranesvia UV radiation. The cross-linked mixed matrix membranes comprisemicroporous materials dispersed in the continuous UV cross-linkedpolymer matrix.

U.S. Pat. No. 8,016,124 disclosed a thin film composite membrane (TFC)comprising a blend of polyethersulfone and aromatic polyimide polymers.The TFC membrane has a layer of a blend of polyethersulfone and aromaticpolyimide with a thickness from about 0.1 to about 3 microns.

U.S. Pat. No. 8,337,598 disclosed a TFC hollow fiber membrane with acore player and a sheath UV-crosslinked polymer layer.

A publication in SCIENCE reported a new type of high permeabilitythermally rearranged polybenzoxazole polymer membranes for gasseparations (Ho Bum Park et al, SCIENCE 318, 254 (2007)). The thermallyrearranged polybenzoxazole membranes are prepared from high temperatureheat treatment of hydroxyl-containing polyimide polymer membranescontaining pendent hydroxyl groups ortho to the heterocyclic imidenitrogen. These polybenzoxazole polymer membranes exhibited extremelyhigh CO₂ permeability (>1000 Barrer) which is similar to that of someinorganic molecular sieve membranes but lower CO₂/CH₄ selectivity thanthat of some small pore inorganic molecular sieve membranes for CO₂/CH₄separation.

Integrally-skinned asymmetric membranes have a selective thin layer anda porous layer from the same membrane material and formed from the samemembrane solution at about the same time. Therefore, the selective thinlayer of an integrally-skinned asymmetric membrane cannot be delaminatedeasily from the non-selective porous layer.

The present invention discloses a high selectivity polyimide membranecomprising a polyimide polymer with hydroxyl and acetoxy functionalgroups, methods for making the membrane, and the use of the membrane fornatural gas upgrading and H₂ purification.

SUMMARY OF THE INVENTION

This invention pertains to a high selectivity polyimide membranecomprising a polyimide polymer with hydroxyl and acetoxy functionalgroups, methods for making the membrane, and the use of the membrane fornatural gas upgrading and H₂ purification. This invention pertains to athin film composite membrane or an asymmetric integrally skinnedmembrane comprising a polyimide polymer with hydroxyl and acetoxyfunctional groups and with either flat sheet or hollow fiber geometry.

The present invention provides a high selectivity polyimide membranecomprising a polyimide polymer with hydroxyl and acetoxy functionalgroups. The incorporation of both hydroxyl and acetoxy functional groupsinto the polyimide polymer in the present invention provides themembrane comprising the polyimide polymer with hydroxyl and acetoxyfunctional groups not only high selectivity, but also highplasticization resistance due to the existence of H-bondings. The molarratio of the hydroxyl groups to the acetoxy groups on the polyimide inthe present invention needs to be in a range of 1:1 to 8:1 in order tomake the polyimide membrane with high selectivity, high permeance, aswell as high plasticization resistance to CO₂ and other condensablegases.

The present invention provides a high selectivity polyimide membranecomprising a polyimide polymer with hydroxyl and acetoxy functionalgroups that comprises a plurality of repeating units of formula (I),wherein formula (I) is

wherein n and m are independent integers from 20 to 500; wherein n:m isin a range of 1:1 to 8:1.

The polyimide polymer with hydroxyl and acetoxy functional groups usedfor making the high selectivity polyimide membrane described in thecurrent invention have a weight average molecular weight in the range of50,000 to 1,000,000 Daltons, preferably between 70,000 to 500,000Daltons.

The high selectivity polyimide membrane comprising a polyimide polymerwith hydroxyl and acetoxy functional groups in the present invention canbe either asymmetric integrally skinned membrane or thin film composite(TFC) membrane.

The asymmetric integrally-skinned flat sheet or hollow fiber highselectivity polyimide membrane comprising a polyimide polymer withhydroxyl and acetoxy functional groups in the present invention wasprepared by a phase inversion process, and then by applying a thincoating layer on the surface of the membrane.

The membrane dope formulation for the preparation of the asymmetricintegrally-skinned flat sheet or hollow fiber polyimide membranecomprising a polyimide polymer with hydroxyl and acetoxy functionalgroups in the present invention comprises good solvents for thepolyimide polymer that can completely dissolve the polymer.Representative good solvents for use in this invention includeN-methylpyrrolidone (NMP), N,N-dimethyl acetamide (DMAC), methylenechloride, N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO),dioxanes, 1,3-dioxolane, acetone, mixtures thereof, others known tothose skilled in the art and mixtures thereof. In some cases, themembrane dope formulation for the preparation of asymmetricintegrally-skinned flat sheet or hollow fiber high selectivity polyimidemembrane comprising a polyimide polymer with hydroxyl and acetoxyfunctional groups in the present invention also comprises poor solventsthat cannot dissolve the polymers such as methanol, ethanol,tetrahydrofuran (THF), toluene, n-octane, n-decane, lactic acid, citricacid, isopropanol, and mixtures thereof. It is believed that the properweight ratio of the solvents used in the present invention providesasymmetric integrally-skinned flat sheet or hollow fiber polyimidemembrane comprising a polyimide polymer with hydroxyl and acetoxyfunctional groups with less than 200 nm super thin nonporous selectiveskin layer which results in high permeances.

The thin film composite polyimide membrane comprising a polyimidepolymer with hydroxyl and acetoxy functional groups described in thecurrent invention comprises a thin nonporous selective separation layercomprising the polyimide polymer with hydroxyl and acetoxy functionalgroups described in the present invention and a porous nonselectivemechanical support layer made from a material different from thepolyimide polymer with hydroxyl and acetoxy functional groups describedin the present invention. The porous nonselective mechanical supportlayer made from a material different from the polyimide polymer withhydroxyl and acetoxy functional groups described in the presentinvention with a low selectivity and high flux can be made frommaterials including cellulose acetate, cellulose triacetate,polysulfone, polyethersulfone, polyamide, polyimide, polyetherimide,polyurethane, polycarbonate, polystyrene, polybenzoxazole, or mixturesthereof.

One asymmetric integrally-skinned hollow fiber polyimide membranecomprising a polyimide polymer with hydroxyl and acetoxy functionalgroups described in the present invention is fabricated from apoly(2,2′-bis-(3,4-dicarboxyphenyl)hexafluoropropanedianhydride-3,3′-dihydroxy-4,4′-diamino-biphenyl-3,3′-diacetoxy-4,4′-diamino-biphenyl)(abbreviated as6FDA-HAB-OH-OAc) synthesized from the condensationreaction of 2,2′-bis-(3,4-dicarboxyphenyl)hexafluoropropane dianhydride(6FDA) with 3,3′-dihydroxy-4,4′-diamino-biphenyl (HAB) in DMAc or NMPpolar solvent by a two-step process involving the formation of thepoly(amic acid) followed by a solution imidization process. Aceticanhydride was used as the dehydrating agent and pyridine was used as theimidization catalyst for the solution imidization reaction during thesecond step synthesis. More importantly, the amount of acetic anhydrideused during the solution imidization reaction was controlled not onlyfor the formation of acetoxy groups via the reaction between thehydroxyl groups and acetic anhydride, but also to achieve desired ratioof hydroxyl/acetoxy groups.

The 6FDA-HAB-OH-OAc asymmetric integrally-skinned hollow fiber polyimidemembrane showed high H₂/CH₄ separation performance with both high H₂permeance of 323 GPU and high H₂/CH₄ selectivity of 193 for H₂/CH₄separation at 50° C. under 5617 kPa feed pressure with 10% H₂ and 90%CH₄ in the feed gas. The 6FDA-HAB-OH-OAc asymmetric integrally-skinnedhollow fiber polyimide membrane also showed high CO₂/CH₄ separationperformance with CO₂ permeance of 88 GPU and CO₂/CH₄ selectivity of 35.6for CO₂/CH₄ separation at 50° C. under 5617 kPa feed pressure with 10%CO₂ and 90% CH₄ in the feed gas.

The invention provides a process for separating at least one gas from amixture of gases using the high selectivity polyimide membranecomprising a polyimide polymer with hydroxyl and acetoxy functionalgroups described herein, the process comprising: (a) providing a highselectivity polyimide membrane comprising a polyimide polymer withhydroxyl and acetoxy functional groups described in the presentinvention which is permeable to said at least one gas; (b) contactingthe mixture on one side of the high selectivity polyimide membranecomprising a polyimide polymer with hydroxyl and acetoxy functionalgroups to cause said at least one gas to permeate the membrane; and (c)removing from the opposite side of the membrane a permeate gascomposition comprising a portion of said at least one gas whichpermeated said membrane.

The high selectivity polyimide membranes comprising a polyimide polymerwith hydroxyl and acetoxy functional groups described in the currentinvention are not only suitable for H₂ purification application, butalso suitable for a variety of other gas separations such as CO₂/CH₄,O₂/N₂, and H₂S/CH₄ separations.

DETAILED DESCRIPTION OF THE INVENTION

The use of membranes for separation of both gases and liquids is agrowing technological area with potentially high economic reward due tothe low energy requirements and the potential for scaling up of modularmembrane designs. Advances in membrane technology, with the continuingdevelopment of new membrane materials and new methods for the productionof high performance membranes will make this technology even morecompetitive with traditional, high-energy intensive and costly processessuch as distillation. Among the applications for large scale gasseparation membrane systems are nitrogen enrichment, oxygen enrichment,hydrogen recovery, removal of hydrogen sulfide and carbon dioxide fromnatural gas and dehydration of air and natural gas. Also, varioushydrocarbon separations are potential applications for the appropriatemembrane system. The membranes that are used in these applications musthave high selectivity, durability, and productivity in processing largevolumes of gas or liquid in order to be economically successful.Membranes for gas separations have evolved rapidly in the past 25 yearsdue to their easy processability for scale-up and low energyrequirements. More than 90% of the membrane gas separation applicationsinvolve the separation of noncondensable gases: such as nitrogen fromair, and hydrogen from nitrogen, argon or methane. Membrane gasseparation is of special interest to petroleum producers and refiners,chemical companies, and industrial gas suppliers. Several applicationsof membrane gas separation have achieved commercial success, includingnitrogen enrichment from air, hydrogen from nitrogen, argon or methane,carbon dioxide removal from natural gas and biogas and in enhanced oilrecovery.

The present invention provides a high selectivity polyimide membranecomprising a polyimide polymer with hydroxyl and acetoxy functionalgroups. This invention also pertains to the application of the highselectivity polyimide membrane comprising a polyimide polymer withhydroxyl and acetoxy functional groups for H₂ purifications such asH₂/CH₄ separation, and also for a variety of other gas separations suchas separations of CO₂/CH₄, H₂S/CH₄, CO₂/N₂, olefin/paraffin (e.g.propylene/propane), and O₂/N₂ separations.

The present invention provides a high selectivity polyimide membranecomprising a polyimide polymer with hydroxyl and acetoxy functionalgroups that comprises a plurality of repeating units of formula (I),wherein formula (I) is

wherein n and m are independent integers from 20 to 500; wherein n:m isin a range of 1:1 to 8:1.

The polyimide polymer with hydroxyl and acetoxy functional groups usedfor making the high selectivity polyimide membrane described in thecurrent invention have a weight average molecular weight in the range of50,000 to 1,000,000 Daltons, preferably between 70,000 to 500,000Daltons.

The high selectivity polyimide membrane comprising a polyimide polymerwith hydroxyl and acetoxy functional groups in the present invention canbe either asymmetric integrally skinned membrane or thin film composite(TFC) membrane.

The asymmetric integrally-skinned flat sheet or hollow fiber highselectivity polyimide membrane comprising a polyimide polymer withhydroxyl and acetoxy functional groups in the present invention wasprepared by a phase inversion process, and then by applying a thincoating layer on the surface of the membrane.

The membrane dope formulation for the preparation of asymmetricintegrally-skinned flat sheet or hollow fiber polyimide membranecomprising a polyimide polymer with hydroxyl and acetoxy functionalgroups with high selectivities for gas separations in the presentinvention comprises good solvents for the polyimide polymer withhydroxyl and acetoxy functional groups that can completely dissolve thepolymers. Representative good solvents for use in this invention includeN-methylpyrrolidone (NMP), N,N-dimethyl acetamide (DMAC), methylenechloride, N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO),dioxanes, 1,3-dioxolane, acetone, mixtures thereof, others known tothose skilled in the art and mixtures thereof. In some cases, themembrane dope formulation for the preparation of asymmetricintegrally-skinned flat sheet or hollow fiber polyimide membranecomprising a polyimide polymer with hydroxyl and acetoxy functionalgroups in the present invention also comprises poor solvents for thepolyimide polymer with hydroxyl and acetoxy functional groups thatcannot dissolve the polymers such as methanol, ethanol, tetrahydrofuran(THF), toluene, n-octane, n-decane, lactic acid, citric acid,isopropanol, and mixtures thereof. It is believed that the proper weightratio of the solvents used in the present invention provides asymmetricintegrally-skinned flat sheet or hollow fiber polyimide membranecomprising a polyimide polymer with hydroxyl and acetoxy functionalgroups with less than 200 nm super thin nonporous selective skin layerwhich results in high permeances.

One asymmetric integrally-skinned hollow fiber polyimide membranecomprising a polyimide polymer with hydroxyl and acetoxy functionalgroups described in the present invention is fabricated from apoly(2,2′-bis-(3,4-dicarboxyphenyl)hexafluoropropanedianhydride-3,3′-dihydroxy-4,4′-diamino-biphenyl-3,3′-diacetoxy-4,4′-diamino-biphenyl)(abbreviated as 6FDA-HAB-OH-OAc) synthesized from the condensationreaction of 6FDA with HAB in DMAc or NMP polar solvent by a two-stepprocess involving the formation of the poly(amic acid) followed by asolution imidization process. Acetic anhydride was used as thedehydrating agent and pyridine was used as the imidization catalyst forthe solution imidization reaction during the second step synthesis. Moreimportantly, the amount of acetic anhydride used during the solutionimidization reaction was controlled not only for the formation ofacetoxy groups via the reaction between the hydroxyl groups and aceticanhydride, but also to achieve desired ratio of hydroxyl/acetoxy groups.The molar ratio of hydroxyl/acetoxy in 6FDA-HAB-OH-OAc is 3:1.

The 6FDA-HAB-OH-OAc asymmetric integrally-skinned hollow fiber polyimidemembrane showed high H₂/CH₄ separation performance with both high H₂permeance of 323 GPU and high H₂/CH₄ selectivity of 193 for H₂/CH₄separation at 50° C. under 5617 kPa feed pressure with 10% H₂ and 90%CH₄ in the feed gas. The 6FDA-HAB-OH-OAc asymmetric integrally-skinnedhollow fiber polyimide membrane also showed high CO₂/CH₄ separationperformance with CO₂ permeance of 88 GPU and CO₂/CH₄ selectivity of 35.6for CO₂/CH₄ separation at 50° C. under 5617 kPa feed pressure with 10%CO₂ and 90% CH₄ in the feed gas.

The thin film composite polyimide membrane comprising a polyimidepolymer with hydroxyl and acetoxy functional groups described in thecurrent invention comprises a thin nonporous selective separation layercomprising the polyimide polymer with hydroxyl and acetoxy functionalgroups described in the present invention and a porous nonselectivemechanical support layer made from a material different from thepolyimide polymer with hydroxyl and acetoxy functional groups describedin the present invention. The thin film composite polyimide membranecomprising a polyimide polymer with hydroxyl and acetoxy functionalgroups described in the current invention has either hollow fiber orflat sheet geometry.

The porous nonselective mechanical support layer was made from amaterial different from the polyimide polymer with hydroxyl and acetoxyfunctional groups described in the present invention with a lowselectivity and high flux. Selection of the porous nonselectivemechanical support layer for the preparation of TFC polyimide membranecomprising a polyimide polymer with hydroxyl and acetoxy functionalgroups in the present invention may be made on the basis of the heatresistance, solvent resistance, and mechanical strength of the porousnonselective mechanical support layer, as well as other factors dictatedby the operating conditions for selective permeation. The porousnonselective mechanical support layer is preferably at least partiallyself-supporting, and in some instances may be essentiallyself-supporting. The porous nonselective mechanical support layer mayprovide essentially all of the structural support for the membrane. Somepreferred polymers different from the polyimide polymer with hydroxyland acetoxy functional groups that are suitable for the preparation ofthe porous nonselective mechanical support layer for the TFC polyimidemembrane comprising a polyimide polymer with hydroxyl and acetoxyfunctional groups according to the present invention include, but arenot limited to, polysulfones, sulfonated polysulfones, polyethersulfones(PESs), sulfonated PESs, polyethers, polyetherimides such as Ultem,cellulosic polymers such as cellulose acetate and cellulose triacetate,polyamides, polyimides such as P84 and P84HT, polyether ketones, andblends thereof.

Some preferred solvents that can be used for dissolving the polyimidepolymer with hydroxyl and acetoxy functional groups for the preparationof TFC polyimide membrane comprising a polyimide polymer with hydroxyland acetoxy functional groups described in the current invention includeNMP, DMAC, methylene chloride, DMF, DMSO, dioxanes, 1,3-dioxolane,acetone, isopropanol, and mixtures thereof. For the preparation of TFCflat sheet polyimide membrane comprising a polyimide polymer withhydroxyl and acetoxy functional groups, it is preferred that thesolution of the polyimide polymer with hydroxyl and acetoxy functionalgroups has a concentration of from about 1 to about 20 wt % to providean effective coating. The dilute solution of the polyimide polymer withhydroxyl and acetoxy functional groups is applied to the surface of theflat sheet porous nonselective mechanical support layer by dip-coating,spin coating, casting, spraying, painting, and other known conventionalsolution coating technologies. For the preparation of TFC hollow fiberpolyimide membrane comprising a polyimide polymer with hydroxyl andacetoxy functional groups, it is preferred that the solution of thepolyimide polymer with hydroxyl and acetoxy functional groups has aconcentration of from about 20 to about 40 wt %. The solution of thepolyimide polymer with hydroxyl and acetoxy functional groups and thepolymer solution for the formation of the porous nonselective mechanicalsupport layer were co-extruded from a spinneret to form TFC hollow fiberpolyimide membrane comprising a polyimide polymer with hydroxyl andacetoxy functional groups.

The invention provides a process for separating at least one gas from amixture of gases using high selectivity polyimide membrane comprising apolyimide polymer with hydroxyl and acetoxy functional groups describedin the present invention, the process comprising: (a) providing a highselectivity polyimide membrane comprising a polyimide polymer withhydroxyl and acetoxy functional groups described in the presentinvention which is permeable to said at least one gas; (b) contactingthe mixture on one side of the high selectivity polyimide membranecomprising a polyimide polymer with hydroxyl and acetoxy functionalgroups described in the present invention to cause said at least one gasto permeate the membrane; and (c) removing from the opposite side of themembrane a permeate gas composition comprising a portion of said atleast one gas which permeated said membrane.

The high selectivity polyimide membrane comprising a polyimide polymerwith hydroxyl and acetoxy functional groups described in the presentinvention is especially useful in the purification, separation oradsorption of a particular species in the liquid or gas phase.

The high selectivity polyimide membrane comprising a polyimide polymerwith hydroxyl and acetoxy functional groups described in the presentinvention is especially useful in gas separation processes in airpurification, petrochemical, refinery, and natural gas industries.Examples of such separations include separation of volatile organiccompounds (such as toluene, xylene, and acetone) from an atmosphericgas, such as nitrogen or oxygen and nitrogen recovery from air. Furtherexamples of such separations are for the separation of CO₂ or H₂S fromnatural gas, H₂ from N₂, CH₄, and Ar in ammonia purge gas streams, H₂recovery in refineries, olefin/paraffin separations such aspropylene/propane separation, and iso/normal paraffin separations. Anygiven pair or group of gases that differ in molecular size, for examplenitrogen and oxygen, carbon dioxide and methane, hydrogen and methane orcarbon monoxide, helium and methane, can be separated using the highselectivity polyimide membrane comprising a polyimide polymer withhydroxyl and acetoxy functional groups described in the presentinvention. More than two gases can be removed from a third gas. Forexample, some of the gas components which can be selectively removedfrom a raw natural gas using the membrane described herein includecarbon dioxide, oxygen, nitrogen, water vapor, hydrogen sulfide, helium,and other trace gases. Some of the gas components that can beselectively retained include hydrocarbon gases. When permeablecomponents are acid components selected from the group consisting ofcarbon dioxide, hydrogen sulfide, and mixtures thereof and are removedfrom a hydrocarbon mixture such as natural gas, one module, or at leasttwo in parallel service, or a series of modules may be utilized toremove the acid components. For example, when one module is utilized,the pressure of the feed gas may vary from 275 kPa to about 2.6 MPa (25to 4000 psi). The differential pressure across the membrane can be aslow as about 70 kPa or as high as 14.5 MPa (about 10 psi or as high asabout 2100 psi) depending on many factors such as the particularmembrane used, the flow rate of the inlet stream and the availability ofa compressor to compress the permeate stream if such compression isdesired. Differential pressure greater than about 14.5 MPa (2100 psi)may rupture the membrane. A differential pressure of at least 0.7 MPa(100 psi) is preferred since lower differential pressures may requiremore modules, more time and compression of intermediate product streams.The operating temperature of the process may vary depending upon thetemperature of the feed stream and upon ambient temperature conditions.Preferably, the effective operating temperature of the membranes of thepresent invention will range from about −50° to about 150° C. Morepreferably, the effective operating temperature of the high selectivitypolyimide membrane comprising a polyimide polymer with hydroxyl andacetoxy functional groups of the present invention will range from about−20° to about 100° C., and most preferably, the effective operatingtemperature of the membranes of the present invention will range fromabout 25° to about 100° C.

The high selectivity polyimide membrane comprising a polyimide polymerwith hydroxyl and acetoxy functional groups described in the presentinvention are also especially useful in gas/vapor separation processesin chemical, petrochemical, pharmaceutical and allied industries forremoving organic vapors from gas streams, e.g. in off-gas treatment forrecovery of volatile organic compounds to meet clean air regulations, orwithin process streams in production plants so that valuable compounds(e.g., vinylchloride monomer, propylene) may be recovered. Furtherexamples of gas/vapor separation processes in which the high selectivitypolyimide membrane comprising a polyimide polymer with hydroxyl andacetoxy functional groups described in the present invention may be usedare hydrocarbon vapor separation from hydrogen in oil and gasrefineries, for hydrocarbon dew pointing of natural gas (i.e. todecrease the hydrocarbon dew point to below the lowest possible exportpipeline temperature so that liquid hydrocarbons do not separate in thepipeline), for control of methane number in fuel gas for gas engines andgas turbines, and for gasoline recovery.

The high selectivity polyimide membrane comprising a polyimide polymerwith hydroxyl and acetoxy functional groups described in the presentinvention also has immediate application to concentrate olefin in aparaffin/olefin stream for olefin cracking application. For example, thehigh selectivity polyimide membrane comprising a polyimide polymer withhydroxyl and acetoxy functional groups described in the presentinvention can be used for propylene/propane separation to increase theconcentration of the effluent in a catalytic dehydrogenation reactionfor the production of propylene from propane and isobutylene fromisobutane. Therefore, the number of stages of a propylene/propanesplitter that is required to get polymer grade propylene can be reduced.Another application for the high selectivity polyimide membranecomprising a polyimide polymer with hydroxyl and acetoxy functionalgroups described in the present invention is for separating isoparaffinand normal paraffin in light paraffin isomerization and MaxEne™, aprocess for enhancing the concentration of normal paraffin (n-paraffin)in the naphtha cracker feedstock, which can be then converted toethylene.

The high selectivity polyimide membrane comprising a polyimide polymerwith hydroxyl and acetoxy functional groups described in the presentinvention can also be operated at high temperature to provide thesufficient dew point margin for natural gas upgrading (e.g, CO₂ removalfrom natural gas). The high selectivity polyimide membrane comprising apolyimide polymer with hydroxyl and acetoxy functional groups describedin the present invention can be used in either a single stage membraneor as the first or/and second stage membrane in a two stage membranesystem for natural gas upgrading.

EXAMPLES

The following examples are provided to illustrate one or more preferredembodiments of the invention, but are not limited embodiments thereof.Numerous variations can be made to the following examples that liewithin the scope of the invention.

Example 1 Preparation of 6FDA-HAB-OH-OAc polyimide dense film membrane

An aromatic polyimidepoly(2,2′-bis-(3,4-dicarboxyphenyl)hexafluoropropanedianhydride-3,3′-dihydroxy-4,4′-diamino-biphenyl-3,3′-diacetoxy-4,4′-diamino-biphenyl)(abbreviated as 6FDA-HAB-OH-OAc) was synthesized from the condensationreaction of 2,2′-bis-(3,4-dicarboxyphenyl)hexafluoropropane dianhydride(6FDA) with 3,3′-dihydroxy-4,4′-diamino-biphenyl (HAB) in DMAc or NMPpolar solvent by a two-step process involving the formation of thepoly(amic acid) followed by a solution imidization process. Aceticanhydride was used as the dehydrating agent and for the reaction withhydroxyl groups on the polyimide polymer chain to achieve a molar ratioof 3:1 for hydroxyl/acetoxy in 6FDA-HAB-OH-OAc polyimide. 4.0 G of6FDA-HAB-OH-OAc was dissolved in 26.0 g of NMP solvent. The mixture wasmechanically stirred for 2 hours to form a homogeneous 6FDA-HAB-OH-OAccasting dope. The resulting homogeneous casting dope was filtered andallowed to degas overnight. The 6FDA-HAB-OH-OAc polyimide dense filmmembrane was prepared from the bubble free casting dope on a clean glassplate using a doctor knife with a 20-mil gap. The membrane together withthe glass plate was then put into a vacuum oven. The solvents wereremoved by slowly increasing the vacuum and the temperature of thevacuum oven. Finally, the membrane was dried at 200° C. under vacuum for48 hours to completely remove the residual solvents to form a polymermembrane in dense film.

Comparative Example to Example 1 Preparation of 6FDA-HAB-OAc polyimidedense film membrane

An aromatic polyimidepoly(2,2′-bis-(3,4-dicarboxyphenyl)hexafluoropropanedianhydride-3,3′-diacetoxy-4,4′-diamino-biphenyl) (abbreviated as6FDA-HAB-OAc) was synthesized from the condensation reaction of 6FDAwith HAB in DMAc or NMP polar solvent by a two-step process involvingthe formation of the poly(amic acid) followed by a solution imidizationprocess. Acetic anhydride was used as the dehydrating agent and for thereaction with hydroxyl groups on the polyimide polymer chain to achieve100% conversion of hydroxyl groups to acetoxy groups with a molar ratioof 0:1 for hydroxyl/acetoxy in 6FDA-HAB-OAc. 6FDA-HAB-OAc dense filmmembrane was prepared using a procedure similar to that for 6FDA-HAB-OAcdense film membrane as described in Example 1.

Example 2 Evaluation of CO₂/CH₄ and H₂/CH₄ separation performance of6FDA-HAB-OH-OAc and 6FDA-HAB-OAc polyimide dense film membranes

The 6FDA-HAB-OH-OAc and 6FDA-HAB-OAc polyimide dense film membranes weretested for CO₂/CH₄ and H₂/CH₄ separations at 50° C. under 791 kPa (100psig) pure gas feed pressure. The results in Tables 1 and 2 show thatthe 6FDA-HAB-OH-OAc polyimide dense film membrane comprising6FDA-HAB-OH-OAc polyimide polymer with 3:1 molar ratio ofhydroxyl/acetoxy has intrinsic CO₂ permeability of 6.44 Barrers (1Barrer=10⁻¹⁰ cm³ (STP) cm/cm² s (cm Hg)) and high single-gas CO₂/CH₄selectivity of 46.7 at 50° C. under 791 kPa for CO₂/CH₄ separation. Thismembrane also has intrinsic H₂ permeability of 24.8 Barrers and highsingle-gas H₂/CH₄ selectivity of 218.8 at 50° C. under 791 kPa forH₂/CH₄ separation. However, the 6FDA-HAB-OAc dense film membranecomprising 6FDA-HAB-OAc polyimide polymer with 0:1 molar ratio ofhydroxyl/acetoxy showed higher permeabilities and much lowerselectivities for CO₂/CH₄ and H₂/CH₄ separations. The significantlyhigher selectivities and reasonable permeabilities for 6FDA-HAB-OH-OAcdense film membrane is due to the control of hydroxyl/acetoxy ratio inthe polymer chain.

TABLE 1 Pure gas permeation test results of 6FDA-HAB-OH-OAc and6FDA-HAB-OAc polyimide dense film membranes for CO₂/CH₄ separation*Dense Film Membrane P_(CO2) (Barrer) α_(CO2/CH4) 6FDA-HAB-OH-OAc 6.4446.7 6FDA-HAB-OAc 7.88 32.7 *P_(CO2) and P_(CH4) were tested at 50° C.and 791 kPa (100 psig); 1 Barrer = 10⁻¹⁰ cm³ (STP).cm/cm².sec.cmHg.

TABLE 2 Pure gas permeation test results of 6FDA-HAB-OH-OAc and6FDA-HAB-OAc polyimide dense film membranes for H₂/CH₄ separation* DenseFilm Membrane P_(CO2) (Barrer) α_(CO2/CH4) 6FDA-HAB-OH-OAc 24.8 218.86FDA-HAB-OAc 28.3 117.5 *P_(H2) and P_(CH4) were tested at 50° C. and791 kPa (100 psig); 1 Barrer = 10⁻¹⁰ cm³ (STP).cm/cm².sec.cmHg.

Example 3 Preparation of 6FDA-HAB-OH-OAc polyimide hollow fibermembranes

A hollow fiber spinning dope containing 13.2 g of 6FDA-HAB-OH-OAcpolyimide, 65.0 g of NMP, 7.4 g of 1,3-dioxolane, and 2.6 g of acetonewas prepared. The spinning dope was extruded at a flow rate of 3.0mL/min through a spinneret at 50° C. spinning temperature. A bore fluidcontaining 10% by weight of water in NMP was injected to the bore of thefiber at a flow rate of 0.6 mL/min simultaneously with the extruding ofthe spinning dope. The nascent fiber traveled through an air gap lengthof 5-7 cm at room temperature, and then was immersed into a watercoagulant bath at 17.9° C. and wound up at a rate of 23-30 m/min. Thewater-wet fiber was annealed in a hot water bath at 85° C. for 30minutes. The annealed water-wet fiber was then sequentially exchangedwith methanol and hexane for three times and for 30 minutes each time,followed by drying at 100° C. in an oven for 1 hour to form6FDA-HAB-OH-OAc polyimide hollow fiber membranes with the spinningconditions listed in Table 3.

TABLE 3 Spinning conditions for 6FDA-HAB-OH-OAc polyimide hollow fibermembranes Dope Bore Take-up Air gap rate rate rate Hollow Fiber Membrane(cm) (mL/min) (mL/min) (m/min) 6FDA-HAB-OH-OAc (G) 3 3.0 0.6 236FDA-HAB-OH-OAc (O) 3 3.0 0.6 30 6FDA-HAB-OH-OAc (P) 5 3.0 0.6 306FDA-HAB-OH-OAc (W) 5 3.0 0.6 23 6FDA-HAB-OH-OAc (Y) 7 3.0 0.6 236FDA-HAB-OH-OAc (GY) 7 3.0 0.6 30

Comparative Example to Example 3 Preparation of 6FDA-HAB-OAc polyimidehollow fiber membranes

A hollow fiber spinning dope containing 28.0 g of 6FDA-HAB-OAcpolyimide, 65.0 g of NMP, 8.8 g of 1,3-dioxolane, and 2.6 g of acetonewas prepared. The spinning dope was extruded at a flow rate of 1.1-2.6mL/min through a spinneret at 50° C. spinning temperature. A bore fluidcontaining 10% by weight of water in NMP was injected to the bore of thefiber at a flow rate of 0.4-0.8 mL/min simultaneously with the extrudingof the spinning dope. The nascent fiber traveled through an air gaplength of 10-15 cm at room temperature, and then was immersed into awater coagulant bath at 17.9° C. and wound up at a rate of 10-23 m/min.The water-wet fiber was annealed in a hot water bath at 85° C. for 30minutes. The annealed water-wet fiber was then sequentially exchangedwith methanol and hexane for three times and for 30 minutes each time,followed by drying at 100° C. in an oven for 1 hour to form 6FDA-HAB-OAcpolyimide hollow fiber membranes with the spinning conditions listed inTable 3.

TABLE 4 Spinning conditions for 6FDA-HAB-OAc polyimide hollow fibermembranes Dope Bore Take-up Air gap rate rate rate Hollow Fiber Membrane(cm) (mL/min) (mL/min) (m/min) 6FDA-HAB-OAc (G) 10 1.1 0.4 106FDA-HAB-OAc (Y) 15 2.6 0.8 23 6FDA-HAB-OAc (P) 10 2.6 0.8 23

Example 4 Evaluation of CO₂/CH₄ separation performance of6FDA-HAB-OH-OAc and 6FDA-HAB-OAc polyimide hollow fiber membranes

The asymmetric integrally-skinned 6FDA-HAB-OH-OAc and 6FDA-HAB-OAcpolyimide hollow fiber membranes were tested for CO₂/CH₄ separation at50° C. under 5617 kPa (800 psig) feed gas pressure with 10% of CO₂ and90% of CH₄ in the feed. The results are shown in Table 5. It can be seenfrom Table 5 that the 6FDA-HAB-OH-OAc polyimide hollow fiber membranescomprising 6FDA-HAB-OH-OAc polyimide with a molar ratio of 3:1 forhydroxyl/acetoxy described in the current invention showed high CO₂permeances of 88-94 GPU and high CO₂/CH₄ selectivities over 30 under5617 kPa high pressure at 50° C. The 6FDA-HAB-OAc polyimide hollow fibermembranes comprising 6FDA-HAB-OAc polyimide without any hydroxyl groupsshowed higher CO₂ permeances of 130 to 137 GPU, but much lower CO₂/CH₄selectivities of less than 22 compared with 6FDA-HAB-OH-OAc polyimidehollow fiber membranes. These results demonstrated that the use of6FDA-HAB-OH-OAc polyimide with controlled hydroxyl and acetoxy groups inthe polymer chain is critical to prepare high selectivity polyimidehollow fiber membranes.

TABLE CO₂/CH₄ separation performance of 6FDA-HAB-OH-OAc and 6FDA-HAB-OAcpolyimide hollow fiber membranes Membrane CO₂ permeance (GPU) CO₂/CH₄selectivity 6FDA-HAB-OH-OAc (Y) 88.2 35.6 6FDA-HAB-OH-OAc (G) 94.0 30.86FDA-HAB-OAc (G) 137.3 21.6 6FDA-HAB-OAc (P) 129.8 19.0 1 GPU = 10⁻⁶ cm³(STP)/cm² s (cm Hg) Testing conditions: 50° C., 5617 kPa (800 psig) feedgas pressure, 10% CO₂ and 90% of CH₄ in the feed.

Example 5 Evaluation of H₂/CH₄ separation performance of 6FDA-HAB-OH-OAcpolyimide hollow fiber membranes

The asymmetric integrally-skinned 6FDA-HAB-OH-OAc polyimide hollow fibermembranes were tested for H₂/CH₄ separation at 50° C. under 4238 kPa(600 psig) feed gas pressure with 10% of H₂ and 90% of CH₄ in the feed.The results are shown in Table 6. It can be seen from Table 6 that the6FDA-HAB-OH-OAc polyimide hollow fiber membranes comprising6FDA-HAB-OH-OAc polyimide with a molar ratio of 3:1 for hydroxyl/acetoxydescribed in the current invention showed high H₂ permeances of over 300GPU and high H₂/CH₄ selectivities of about 185 under 4238 kPa highpressure at 50° C.

TABLE 6 H₂/CH₄ separation performance of 6FDA-HAB-OH-OAc polyimidehollow fiber membranes Membrane H₂ permeance (GPU) H₂/CH₄ selectivity6FDA-HAB-OH-OAc (Y) 356.9 185.2 6FDA-HAB-OH-OAc (GY) 333.9 185.3 1 GPU =10⁻⁶ cm³ (STP)/cm² s (cm Hg) Testing conditions: 50° C., 4238 kPa (600psig) feed gas pressure, 10% H₂ and 90% of CH₄ in the feed.

The invention claimed is:
 1. A process for separating at least one gasfrom a mixture of gases using a polyimide membrane comprising apolyimide polymer with hydroxyl and acetoxy functional groups the molarratio of said hydroxyl functional groups to said acetoxy functionalgroups is in a range of 1:1 to 8:1, said process comprising: (a)providing said polyimide membrane comprising said polyimide polymer withhydroxyl and acetoxy functional groups which is permeable to said atleast one gas; (b) contacting the mixture on one side of the polyimidemembrane to cause said at least one gas to permeate the membrane; and(c) removing from the opposite side of the membrane a permeate gascomposition comprising a portion of said at least one gas whichpermeated said polyimide membrane; wherein said polyimide polymercomprises a plurality of repeating units of formula (I), wherein formula(I) is

wherein n and m are independent integers from 20 to 500; wherein n:m isin a range of 1:1 to 8:1.
 2. The process of claim 1 wherein said mixtureof gases comprises a mixture of carbon dioxide and methane.
 3. Theprocess of claim 1 wherein said mixture of gases comprises a mixture ofhydrogen and methane.
 4. The process of claim 1 wherein said mixture ofgases comprises a mixture of helium and methane.
 5. The process of claim1 wherein said mixture of gases comprises a mixture of at least onevolatile organic compound and at least one atmospheric gas.
 6. Theprocess of claim 1 wherein said mixture of gases comprises nitrogen andhydrogen.
 7. The process of claim 1 wherein said mixture of gasescomprises a mixture of at least two gases selected from the groupconsisting of carbon dioxide, oxygen, nitrogen, water vapor, hydrogensulfide, helium and methane.
 8. The process of claim 1 wherein saidmixture of gases comprises a mixture of volatile organic compounds andat least one atmospheric gas.
 9. The process of claim 7 wherein saidvolatile organic compounds are selected from the group consisting oftoluene, xylene and acetone.
 10. The process of claim 1 wherein saidmixture of gases comprises a mixture of olefins and paraffins.
 11. Theprocess of claim 1 wherein said mixture of gases comprises a mixture ofhydrocarbons and hydrogen.
 12. The process of claim 1 wherein saidprocess is at a temperature from about 20° to about 100° C.
 13. Theprocess of claim 1 wherein said mixture of gases comprises a mixture ofisoparaffin and normal paraffin.
 14. The process of claim 1 wherein saidpolyimide membrane is used in either a single stage membrane system orin either stages of a two stage membrane system.